Aluminum reflector surface and method of producing the same



UNITED STATES PATENT OFFICE ALUMINUM nEFLEcToa SURFACE AND METHOD OF PRODUCING THE SAME Junius D. Edwards, Oakmont, Cyril S. Taylor, New Kensington, and Welker Wallace Wentz, Pittsburgh, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application January 25, 1932,

' Serial No. 588,843

Claims. (Cl. ss-- 1) This invention relates to improvements in the mercial grade of aluminum suitable for this purmetal used for the production of aluminum repose and simple alloys thereof containing at least fleeting surfaces and it is particularly concerned about 98 pe ce t o um.

with the production of etched aluminum reflect- We have found that the etching characteristics 5 ing surfaces of maximum brightness and uniform of aluminum are affected by. th Composition of 5 reflecting qualities. the metal used with respect to certain constituents Etched aluminum surfaces hav found w de present among the impurities, heretofore regardapplication in the production of articles having ed as unimportant and immaterial i dete mingood light reflecting surfaces, particularly where s t e properties of the metal- W ave also in a more or less diffuse reflection is desired. For u d t y controlling the p t on of the 1 example, aluminum and its alloys hav b d aluminum with respect to these constituents the widely in the production of such articles as enetching oharaoteristios may be s improved a graved name plates, electric light reflectors, and reflecting surfaces of maximum ess and more recently for reflectors used in connection uniform reflectivity may be obtained directly y with ultra-violet light reflection, Th type of the action of the usual etching solutions on the face generally desired for these uses is obtained metal- Among the impurities p s n the by treating the aluminum with one or more solualuminum is generally in d a v y sm p rtions having a solvent action on the aluminum centage f pp r nd a r latively lar p r entsurface, whereby a matte or satin finish is proa o silicon- We have found that the p p ty 25 duced. Undesirable results are sometimes obof the aluminum of ta in a uniform etch varies tained, however, because of an irregular action With the percentage of pp p esent and that of the etching solution on the metal so that a aluminum Containing more than a minimum P surface having a streaked or spotted appearance ntage of copper, which is definite though y is obtained. The brightness of the reflecting sursmall, is characterized by the p p y o being face 1 ,1 impaired 1; times by t presence of uniformly etched with the usual etching solutions. 25 gray film left thereon after the etching action. The y film l on the aluminum surface y While th film under m conditions, be the action of the etching solutions we have found readily rubbed off, the rubbing tends to reduce to be due to the P e in the metal of free the brightness of the etched surface and is obelementary d sso ved silicon known in the jectionable in that it requires an additional step art as graphitio s icon. 30 in the preparation of suitable light reflecting sur- The copper p nt a an pu y in he alumifaces. num varies in amount between about 0.01 per cent It is an object of this invention to overcome the and P cent but it generally occurs in difficulties in the preparation of etched aluminum amounts s then P Cent- We ave found reflecting surfaces and to produce an aluminum that in order to obtain a uniformly etched-=-sur- 85 or aluminum alloy reflector metal of improved face on aluminum or its alloys it is necessary that etching qualities. the aluminum contain at least about 0.06 per cent It is a further object of this invention to proand preferably ss t p cent pp a d duce an aluminum or aluminum alloy reflector W ave been able to produce the best results sheet upon which etched reflecting surfaces of with aluminum containing between 0.1 per cent 40 uniform reflectivity and maximum brightness and 0.2 per cent copper. With a copper content may b produced, lying within these limits the etched surface pro- It is more particularly an object of this invenduced on the aluminum by means of ordinary tion to produce etched aluminum reflecting suretching procedure has a regular matte finish enfaces of uniform reflectivity and maximum tirely free from streaks and spots due to the 45 brightness. presence of unetched or poorly etched areas. The

In the production of etched light reflecting surreflection characteristics of this surface are confaces, a commercial grade of aluminum containsequently more uniform and a better appearance ing not more than about 1 per cent of total imand distribution of light are obtainable with it.

purities or a simple alloy thereof containing at The silicon present as an impurity in the alumi- 50 least about 98 per cent of aluminum and a small num of the grade used for this purpose is genpercentage of other elements such as manganese erally less than about half of the total impurities or titanium may be used. The term aluminum present and in most cases varies between about is therefore used herein and in the appended 0.1 per cent and 0.4 per cent. This silicon may be claims in a generic sense to include both a compresent either in solution in the aluminum or in 55 combination with one of the other impurities, such as iron, or as undissolved uncombined graphitic silicon. Silicon present in the dissolved or in the combined state does not materially affect the adaptability of the aluminum to the production of etched light reflecting surfaces. Graphitic silicon, however, leaves the gray film heretofore referred to and its presence in the metal tends to reduce the brightness of the etched surface obtainable on the metal. Graphitic silicon may be caused to dissolve in the aluminum by annealing the metal at suitable temperatures. The anhealing of aluminum used for the production of etched light reflecting surfaces is customarily carried out at temperatures of about 600 to 650 Fahrenheit. We have found, however, that annealing at this temperature, even when carried on for relatively long periods, does not eliminate this gray film produced by etching, but that if the annealing be carried on at temperatures between 800 Fahrenheit and 900 Fahrenheit, pref erably at about 850 Fahrenheit, the silicon may be caused to dissolve with elimination of the gray film attributed to the silicon. The annealing may be followed, of course, by further cold working to change the gage or shape of the aluminum or to increase its hardness. There is thus obtained aluminum free from graphitic silicon which may be etched with the usual solutions without the production of a gray film on the etched surface.

Consequently, we are able, by controlling the copper content of the aluminum so that at least 0.06 per cent and preferably less than 0.3 per cent is present and annealing the metal at a temperature preferably about 850 Fahrenheit, to produce a metal which etches uniformly and which acquires a surface free from any dark film. Thus we are able to produce etched aluminum light reflecting surfaces of uniform reflectivity and maximum brightness directly from the etching bath.

The etching of our improved aluminum light reflecting material may be carried out with most of the ordinary etching solutions. We prefer, however, to use certain solutions in order to obtain the maximum reflectivity. For this purpose we have found two types of etching treatment to be particularly satisfactory; that is, etching with hydrofluoric acid and nitric acid or etching with a solution of sodium hydroxide and sodium fluoride followed by nitric acid. When the etching is carried out using hydrofluoric acid and nitric acid the aluminum is first dipped into a solution of one part concentrated hydrofluoric acid in 19 parts of water at a temperature of 50 to 60 centigrade, until an etch of the desired depth is obtained. The surface is washed and the article is immersed for several seconds in a solution of nitric acid containing one volume of acid to one volume of water and held at room temperature. The aluminum is washed and dried and a clean, bright and uniformly etched surface is obtained. In the sodium hydroxide-sodium fluoride etching procedure a 5 per cent sodium hydroxide solution in water containing about 4 per cent sodium fluoride is used. The aluminum is immersed in this solution at a temperature of about 90 centigrade until the desired etch is obtained. It is then removed, washed, and treated with a 1: 1 nitric acid solution, washed and dried as before. Again a very satisfactory clean, bright, and uniformly etched surface is obtained. It should be noted that the presence of copper in the aluminum causes the metal to turn gray to black on immersion in the hydrofluoric acid or the sodium hydroxide solutions. This black coloration, due to copper, is removed by immersion in the nitric acid. However, the nitric acid does not remove the gray film due to graphitic silicon, if it is present, and this must either be removed by rubbing or prevented from forming.

The effect of the presence of a sufficient amount of copper in aluminum on its etching properties is pointed out specifically by the following examples: A sample of a commercial grade of aluminum containing about 1.0 per cent of impurities, including 0.6 per cent iron, 0.3 per cent silicon, and 0.01 per cent copper, when etched with hydrofluoric and nitric acids as above described, produced a surface which was irregularly etched, having a streaked appearance. On the other hand, a sample of aluminum containing 0.6 per cent iron, 0.08 per cent copper, and 0.18 percent silicon as impurities, when etched in a similar manner, produced a very satisfctory uniform reflecting surface.

The following examples further point out the beneficial effects of copper in the aluminum and at the same time show the results obtainable by annealing the aluminum at 850 Fahrenheit. Samples of commercially pure aluminum containing about 0.44 per cent iron and 0.29 per cent silicon, to which copper had been added to an amount of about 0.1 per cent, were prepared. One sample of this metal was annealed at 650 Fahrenheit according to the usual practice and subsequently etched by the hydrofluoric acidnitric acid. method. An etched surface was produced which was lightly coated with a gray film. When this film was removed the etched surface was found to be regular and uniform. A portion of the silicon was present as graphitic silicon as shown by qualitative tests. The other sample of this metal was annealed at 850 Fahrenheit and etched with hydrofluoric and nitric acids. This treatment produced a bright uniformly etched surface, free from any film.

We claim? 1. A method of producing an aluminum article having a uniformly etched light-reflecting surface comprising forming the article of aluminum containing 0.06 to 0.3 per cent of copper and a substantial amount of silicon, annealing the aluminum at a temperature of 800 to 900 F. until all elementary silicon contained therein is dissolved and subsequently treating the surface of said article with a solvent for aluminum and non-graphitic silicon and a solvent for copper.

2. A method of producing an aluminum article having a uniformly etched light-reflecting surface of maximum brightness comprising treating the surface of an article formed of aluminum containing 0.06 to 0.3 per cent of copper and free from undissolved elementary silicon with a solvent for aluminum and non-graphitic silicon and a solvent for copper.

3. A method of producing an aluminum article having a uniformly etched light-reflecting surface comprising treating the surface of an article formed of aluminum containing 0.06 to 0.3 per cent of copper with a solvent for aluminum and a solvent for copper.

4. In a method of producing aluminum lightrefiecting surfaces of uniform reflectivity and maximum brightness, the steps comprising annealing aluminum containing 0.06 to 0.3 per cent of copper and a substantial amount of silicon at a temperature of 800 to 900 F. until all uncombined silicon contained therein is dissolved and subsequently etching the surface of the alumi- 0.3 per cent of copper and substantially free from undissolved elementary silicon to dissolve therefrom aluminum, non-graphitic silicon and copper.

'6. A method of producing an aluminum article having a uniformly etched light-reflecting surface comprising forming the article of aluminum containing 0.06 to 0.3 per cent of copper and a substantial amount of silicon, annealing the aluminum at a temperature of 800 to 900 F. until all the elementary silicon contained therein is dissolved and subsequently etching the surface of said article with hydrofluoric and nitric acids.

7. A method of producing an aluminum article having a uniformly etched light-reflecting surface comprising forming the article of aluminum containing 0.06 to 0.3 per cent of copper and a substantial amount of silicon, annealing the aluminum until all elementary silicon contained therein is dissolved and subsequently treating the surface of said article with a solvent for aluminum and non-graphltic silicon and a solvent for copper.

8. As an article of manufacture, an aluminum diffuse light-reflecting means formed by etching aluminum containing,0.06 to 0.3 per cent of copper to produce a diffuse light-reflecting surface substantially free from unetched and partially etched areas, which is visibly uniform.

9. As an article of manufacture, an aluminum diffuse light-reflecting means formed by etching aluminum containing 0.06 to 0.3 per cent copper and free from undissolved silicon to produce a diffuse light-reflecting surface substantially free from unetched and partially etched areas and free from film, which is visibly uniform.

10. As an article of manufacture, an aluminum diffuse light-reflecting means formed by etching aluminum containing 0.1 to 0.2 per cent copper to produce a diffuse light-reflecting surface substantially free from unetched and partially etched areas, which is visibly uniform.

' JUNIUS D. EDWARDS. CYRIL S. TAYLOR.

WELKER WALLACE WENTZ. 

